[clang] Add Clang attribute to ensure that fields are initialized explicitly (PR #102040)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

> Huh, that's strange. Any idea why the warning isn't being emitted?

Yeah, that is really kind of weird; the logic looks correct to me. You'll probably have to look at it under the debugger to see what's going on. :-(

https://github.com/llvm/llvm-project/pull/102040